A former IT company employee admitted in court Thursday to sending online death threats by hijacking other people's computers and changed his plea to guilty in a high-profile case in which four people were wrongly arrested.

At the Tokyo District Court, Yusuke Katayama, 32, had initially denied sending messages in 2012 threatening to attack elementary schools and claiming to have planted bombs on an airplane and at a shrine, arguing that he, too, had been wrongly arrested.

"I'm truly sorry," Katayama told the court. "I have deceived a lot of people up to now. I betrayed everyone, including those I threatened and those wrongly arrested."